The present invention pertains to acoustic charge transport (ACT) devices and more particularly to an ACT device which improves device performance by reducing device insertion loss and achieving greater device selectivity when operated as a filter.
An ACT device employs a powerful UHF surface acoustic wave (SAW) propagating on the top, highly polished surface of a wafer of piezoelectric semiconductor material. The ACT bunches mobile charge carriers in the extrema of the SAW electrical potential and transports these discrete charge packets at the speed of sound through the semiconductor material. This process is described in detail in U.S. Pat. No. 4,633,285, entitled "Acoustic Charge Transport Device and Method", issued to Billy Jo Hunsinger and Michael Joseph Hoskins. The SAW thus functions similarly to the clocking signal in a conventional charge-coupled device (CCD), but without the complex interconnections which CCD's require.
A conventional ACT device will employ a single input ohmic contact for injecting charge carriers into the semiconducting channel of the ACT device. This results in very high input impedances to the ACT device relative to that of conventional wide-band 50 .OMEGA. systems. In turn, large insertion losses occur.
The use of a single input contact also places the full burden of achieving a filter transfer function on the output nondestructive sensing (NDS) array. As the charge packets travel beneath the NDS array, the charge packets in the channel are capacitively coupled to the NDS array elements. Samples of the channel charge packets are formed in the NDS without disturbing the charge packets in the channel. These samples are individually weighted by any of several means and then summed, forming a transversal filter structure.
A single input contact structure will only allow charge to be injected at one point along the length of the channel. This means that a transversal filter function cannot be realized at the input contact structure. The entire filter transfer function is achieved at the NDS output contact structure alone. Thus the overall device selectivity and ultimate out-of-band rejection of unwanted, interfering signals is limited to that which can be realized by the NDS output contact alone.
Accordingly, it is an object of the present invention to provide an acoustic charge transfer device which reduces the input impedance of input contact structures of the device and thereby reduces the insertion losses realized when such ACT device is operated in a conventional 50 .OMEGA. system.
It is a further object of the present invention to achieve improved filter selectivity and out-of-band rejection of interfering signals by the acoustic charge transfer devices, compared to the constraints imposed when filter functions are achieved by the NDS array alone.